Variables
Const event
event: "http://purl.org/NET/c4dm/event.owl#" = "http://purl.org/NET/c4dm/event.owl#"
Const ical
ical: "http://www.w3.org/2002/12/cal/ical#" = "http://www.w3.org/2002/12/cal/ical#"
Const time
time: "http://www.w3.org/2006/time" = "http://www.w3.org/2006/time"